Flame-proof,moisture resistant coated article and process of making same

ABSTRACT

A FLAME-PROOF, MOISTURE RESISTANT COATING SYSTEM ESPECIALLY SUITABLE FOR PROVIDING COATINGS ON ELECTRICAL RESISTORS WHICH MEET THE MIL. STD. REQUIREMENTS FOR MOISTURE RESISTANCE COMPRISING THREE COATING COMPOSITIONS: SAID FIRST COMPOSITION COMPRISING A HIGHLY INORGANIC SILICONE RESIN, AN INORGANIC FILER AND A SOLVENT; SAID SECOND COATING COMPOSITION COMPRISING AN AT LEAST PARTIALLY HYDROLYZED TETRAALKYL ORTHOSILICATE, SILICON DIOXIDE, ALUMINUM OXIDE, INORGANIC PIGMENTS, A SUSPENSION AGENT AND A SOLVENT; SAID THIRD COATING COMPOSITION C OMPRISING AN AT LEAST PARTIALLY HYDROLYZED TETRALKYL ORTHOSILICATE, PREDOMINANTLY INORGANIC SILICONE RESIN, SUSPENDED RESINOUS PARTICLES AND A COMPATIBLE SOLVENT. THE INVENTION ALSO INCLUDES A PROCESS FOR APPLYING THESE COATING SYSTEMS TO AN ELECTRICAL RESISTOR COMPRISING SEPARATELY APPLYING SAID FIRST, SECOND AND THIRD COATING COMPOSITIONS IN THE STATED ORDER AND CURING. THE INVENTION ALSO INCLUDES ELECTRICAL RESISTORS COATED WITH THE ABOVE-DESCRIBED COATING SYSTEM.

United States Patent O U.S. Cl. 117-218 29 Claims ABSTRACT OF THE DISCLOSURE A flame-proof, moisture resistant coating system especially suitable for providing coatings on electrical resistors which meet the Mil. Std. requirements for moisture resistance comprising three coating compositions: said first composition comprising a highly inorganic silicone resin, an inorganic filler and a solvent; said second coating composition comprising an at least partially hydrolyzed tetraalkyl orthosilicate, silicon dioxide, aluminum oxide, inorganic pigments, a suspension agent and a solvent; said third coating composition comprising an at least partially hydrolyzed tetraalkyl orthosilicate, predominantly inorganic silicone resin, suspended resinous particles and a compatible solvent. The invention also includes a process for applying these coating systems to an electrical resistor comprising separately applying said first, second and third coating compositions in the stated order and curing. The invention also includes electrical resistors coated with the above-described coating system.

BACKGROUND OF THE INVENTION Electrical resistors find wide and diversified applications in modern-day technology. Recently, electrical film resistors have come into wide use. Normally, these film resistors comprise a substrate such as glass coated with a thin film of resistor material, such as tin oxide. Various coating compositions for providing protective surfaces for these resistors have been suggested. The prior art is faced with the problem, however, that these conventional protective coatings very often flame and burn and are destroyed due to the heat produced from severe overloads on the resistor. This burning of the resistor coating results not only in the destruction of the resistor itself, but very often in damage to adjacent elements in the system in which it is employed.

Moreover, many of the conventional coatings are soft or brittle, having little resistance to chipping, abrasion, etc. In addition, many of the conventional coatings have a low degree of arc resistance and fail to permit an opening of the circuit on severe overload.

Also, all of the previously suggested coating compositions for electrical resistors do not meet the Mil. Std. requirements for moisture resistance. Consequently, electrical resistors coated with the prior art compositions are highly susceptible to damage resulting from moisture absorption.

It is an object of the present invention to provide a coating for electrical resistors which meets the Mil. Std. requirements for moisture resistance.

It is a further object of the present invention to provide a coating for electrical resistors which is highly resistant to arcing and permits opening on severe overload of the circuit in which the resistor is situated.

It is a further object of the present invention to provide a coating for electrical resistors which is flame-proof.

It is a further object of the present invention to provide a coating for electrical resistors which is strong and resistant to chipping, abrasion, etc.

It is a further object of the present invention to provide electrical resistors having flame-proof, moisture resistant coatings.

It is a further object of the present invention to provide a process for providing flame-proof, moisture resistant coatings on electrical resistors.

BRIEF DESCRIPTION OF THE INVENTION The coating system of the present invention comprises three coating compositions; said first composition comprising a highly inorganic silicone resin, an inorganic filler and a solvent; said second coating composition comprising an at least partially hydrolyzed tetraalkyl orthosilicate, silicon dioxide, aluminum oxide, inorganic pigments, a suspension agent and a solvent; said third coating composition comprising an at least partially hydrolyzed tetraalkyl orthosilicate, highly inorganic silicone resin, suspended resinous particles and a compatible solvent.

The invention also comprises electrical resistors containing a flame-proof, moisture resistant, arc resistant, chip and abrasion resistant coating, prepared by separately applying the above-described three coating compositions in the stated order and effecting a cure of the individual coatings.

DETAILED DESCRIPTION OF THE INVENTION Applicant has found that the above-described three coating compositions, when applied to an electrical resistor, have a synergistic-like effect upon each other in imparting flame-proof, moisture resistant, arc resistant and abrasion and chip resistant properties to the resistor. 'In other words, whereas one of the coating compositions may impart a high degree of moisture resistance to the resistor when used alone, it is also highly susceptible to burning or producing flammable smoke upon severe overload. When used in combination with one or both of the other coating compositions, however, its susceptibility to burning is eliminated while its ability to provide a moisture resistant protective surface is not impaired. Moreover, one or both of the other coating compositions used in conjunction therewith, while improving the flame-proof qualities of the first coating, themselves have a low degree of moisture resistance. The third coating composition, for example, acts in a synergistic-like man ner to improve the moisture resistant properties of the second coating composition.

The first coating composition comprises a highly inorganic silicone resin, an inorganic filler and a solvent. This composition, upon application to an electrical resistor, provides an internal, highly moisture resistant barrier coating. This coating also has good dielectric strength and is arc resistant.

The highly inorganic silicon resin component may be any silicone resin containing relatively little organic substitution. Generally, these silicones are derived from diand tri-functional silanes represented by the following respective structural formula:

These silanes interact through their alkoxy groups (via hydrolysis) to produce silicone resins containing the following recurring units:

0-si L I .1 R and R will complete the valences of the Si atom in the resin where the starting silane is a di-functional silane (containing 2 al-koxy groups). R and, to a large extent, a cross or bridging link with another silicone resin chain will complete the valences of the Si atom 3 Where the starting silane is a tri-functional silane. It is to be understood that mixtures of diand tri-functional silanes may be employed to prepare the silicone resins.

By highly inorganic silicone resin is meant one in which the Si and content is at least about 50% (as SiO by weight of the resin.

In the above structural formulas, in the functional alkoxy groups, R R and R may be the same or different and are lower alkyl groups such as methyl, ethyl, propyl, etc.

R and R which remain in the silicone resin as the main organic constituents are selected such that the Si and 0 content of the resin is maintained at at least the 50% (as SiO by Weight value specified above. R and R may be the same or different, and are preferably lower alkyl groups such as methyl, ethyl, nor iso-propyl, nor iso-butyl, and phenyl, etc.

It is critical to the practice of the invention that the silicone resin be highly inorganic. Silicone resins have been employed in the past as ingredients in coating compositions for resistors, etc. It is theorized, however that because of their high organic material content and structure, their decomposition products have a low flash point and, upon being subjected to the elevated temperatures (approximately 600 C.) caused by severe overloads on resistors, tend to flame or burn easily giving rise to the above discussed disadvantages.

Apparently the highly inorganic silicone resins of the present invention decompose at elevated temperatures to yield relatively low amounts of decomposition products at a slow rate which have a high flash point and which will not flame or burn at the temperature existing in cases of severe overload. It is to be understood, however, that I do not intend to be bound by this theory.

The most preferred silicone resins are the polymethyl phenyl siloxanes which can be prepared either from the difunctional silanes illustrated above wherein R is methyl and R is phenyl or from two trifunctional silanes wherein R ise methyl in one and phenyl in the other. It has been found that this resin, because of its mixed organic content, decomposes slowly over a wide temperature range thereby lowering the amount of available decomposition products available for potential flaming at any one point during the decomposition process. In other words, the methyl and phenyl components each decompose at a different temperature, giving rise to the presence of less decomposition products at any specified temperature and point of time during the decomposition cycle than if both R and R were identical.

Another disadvantage associated with the presence of relatively large amounts of organic decomposition products is that these materials cause the partly decomposed coating to become electrically conductive at elevated temperatures. Accordingly, resistors having coatings containing these products will not open the circuit on severe overload.

The amount of silicone resin in the first coating composition may vary from about 5 to about 65 percent. The utilization of less than about 5 percent results in a coating having poor moisture resistance. Employing more than 65 percent of the silicone resin produces a composition which is difficult to handle and process.

The inorganic filler may be any of those conventionally used as fillers. Generally preferred are the silicates. The amount of silicate filler may vary from about 1 to about percent. The amount of silicate affects the adhesion to the first coating layer of the subsequently applied coating compositions. The employment of too little silicate produces cracks in the subsequently applied coating layer. The employment of more than 10 percent of silicate reduces the moisture resistant properties of the overall system.

The silicate filler material is employed in powdered form. The particles may vary in size from about 80 mesh to sub-micron.

The most preferred silicate fillers are the micas. Mica particles take the form of small platelets. I have found that these platelets align themselves in the coatings produced from the compositions of the invention to form a virtually impervious moisture barrier or shield. Although other conventional inorganic fillers aid in improving the temperautre resistance, adhesion, etc., of the coating compositions, the micas have been found to be particularly effective because of their platelets form particles.

The solvent employed may be any liquid which is inert to the ingredients of the composition and compatible with the overall coating system. Among the suitable solvents may be mentioned 'ketones such as acetone, methylcthylketone, etc., aliphatic alcohols such as ethanol, propanol, isopropanol, diacetone alcohol, etc., halogenated hydrocarbons such as triand tetrachloroethylene, etc.; hydrocarbons such as toluene, xylene, benzene, isooctane, hexane, etc.; dimethyl formamide, dimethyl sulfoxide, N-methyl pyrrolidone, dioxane, etc. The preferred solvent is isopropanol.

Generally, the amount of solvent may vary from about 20 to about 70 percent. The amount of solvent employed in each particular instance will depend to a large extent, of course, on the amounts of silicone resin and silicate filler employed.

The second coating composition comprises an at least partially hydrolyzed tetraalkyl orthosilicate, silicon dioxide, aluminum oxide, inorganic pigments, a suspension agent and a solvent. This composition constitutes the main portion of the overall coating system and imparts shape and non-flammability to the system. This coating acts as an insulator and inhibits external arcing while aiding in opening the circuit on overload.

The second coating composition or insulator is the subject matter of my copending applications, Ser. Nos. 724,220 and 724,271, both filed on April 25, 1968. It is to be understood that the disclosures of these applications are incorporated herein by reference.

Suitable as the hydrolyzed tetraalkyl orthosilicate component are the methyl, ethyl, propyl and butyl orthosilicates. The most preferred is a partially hydrolyzed tetraethyl orthosilicate.

The degree of hydrolysis of the partially hydrolyzed tetraalkyl orthosilicate is not overly critical. Generally, those orthosilicates having a higher degree of hydrolysis have been found more desirable. The degree of hydrolysis may range from about 10 percent to about percent. The amount of the hydrolyzed tetraethyl orthosilicate employed in the coating composition may range from about 5 to about 55 percent.

The amount of aluminum oxide may range from about 4 to about percent. The amount of silicon dioxide may range from about 1 to about 48 percent.

Any of the conventional suspension agents may be employed in the second coating composition. Examples of suitable suspension agents include the organic derivatives to the various montmorillonites such as their alkyl ammonium derivatives, etc., commonly known as the Bentones. Other suspension agents include clays such as the montmorillonites themselves, diatomaceous earths such as kieselguhr, etc., and amorphous silica. The most preferred suspension agents are the Bentones. Generally, the amount of suspension agent may vary from about 0.1 to about 2 percent.

Any inert liquid which is compatible with the other ingredients in the overall coating system may be employed as the solvent. Suitable solvents include the lower alkanols such as methanol, ethanol, isopropanol, etc., methyl ethyl ketone, dimethyl formamide, diacetone alcohol, dimethylsulfoxide, trichlorethane, trichloroethylene, tetrachloroethylene, N methyl pyrrolidone, isooctane, hexane, benzene, dioxane, etc. The amount of solvent employed may vary from about 0 to about 20 percent, depending in each particular instance upon the amounts of the remaining ingredients incorporated in the composition.

If desired, any of the conventional inorganic fillers and pigments may be added to the second coating composition. It is especially preferred to add titanium di-' oxide to the composition which is a functional pigment and filler, i.e., it aids in the processing and handling of the composition. Other inorganic pigments may be utilized in conjunction with the titanium dioxide, Suitable pigments include cobaltous aluminate, chromium oxide, Fe O Fe O and lampblack. Generally, an amount of titanium dioxide ranging from about 1 to about 45 percent may be employed. If desired, amounts of other pigments ranging from about 0.5 to about percent may be employed.

In my co-pending application Ser. No. 724,271, the desirability of utilizing a non-cowled suspension agent and titanium dioxide in relatively large particle agglomerate size form (i.e., greater than 1 micron) is set forth. The desirability of employing ingredients in the composition such that a cured coating contains less than 0.05% Na, less than 0.05% K, less than 0.01% Li, less than 0.001% Cs and less than 0.001% Rb is also set forth in that application.

In my co-pending application Ser. No. 724,220 the desirability of including crystalline silicon dioxide having a particle size of between 100 and 325 mesh in the coating composition is set forth. Briefly, it has been found that this inhibits the tendency of the coating composition to crack during and after curing of the composition.

Again, as noted above, it is to be understood that the disclosures of these applications are incorporated herein by reference.

A hardening agent may also be added to the second coating composition. Suitable hardening agents are the alkyl borates, particularly trimethylborate and carbon tetrachloride. A preferred hardening agent is the methanol azeotrope of trimethylborate. Amounts up to about 3 percent of the hardening agent may be added to the composition.

The third coating composition impregnates and protects the second coating composition upon application to a electrical resistor and renders the overall unit chip, abrasion, and dirt resistant. The third coating also enhances the moisture resistance of the system. The third coating composition is the subject matter of my copending application Ser. No. 688,974, filed Dec. 8, 1967. It is to be understood that the disclosure of this application is incorporated herein by reference.

The hydrolyzed tetraalkyl orthosilicate component of the third coating composition may be the same or different from that employed in the second composition. The silicone resin employed in the third coating composition may be the same or different from that employed in the first coating composition. The amount of hydrolyzed tetraalkyl orthosilicate may range from about 1 to about 95 percent in the third coating composition. The amount of silicone resin may vary from about 1 to about 90 percent.

The resinous material may be any resin having a low coetficient of friction. Where a high degree of flame, heat and arc resistance is desired, however, it is preferred to employ suspensions of the fluorocarbon resins such as those of tetrafluoroethylene, trifluorochloroethylene, fluorinated ethylene-propylene resins, vinylidene fluoride, copolymers of tetrafluoroethylene and hexafiuoropropylene vinyl fluoride and telomers of tetrafluoroethylene.

Where a high degree of flame, arc resistance and also heat resistance is not desired, any resinous material having a coefficient of friction (static) varying from about 0.04 to about 1.00 may be employed. Suitable nonfluorinated resins, for example, are polyethylene and polypropylene.

The resinous particles are preferably suspended in a carrier liquid prior to incorporation in the third coating composition; however, the resinous material may be suspended in the solvent employed in the third composition. The only requirements for the liquid carrier are that it be one in which the resin is insoluble and that it be compatible with the other ingredients of the composition. Suitable carrier liquids are the aromatic liquids such as toluene, benzene, xylene, etc. Generally, the amount of the resin suspension may vary from about 1 to about 75 percent. If a pre-formed suspension of the resin in the carrier liquid is employed, the amount of resin in the suspension may vary from 0.1 to 52%, thus, the amount of resin utilized thus varies from 0.1% to 39%, this figure being derived by multiplying the amount of resin in the suspension by the amount of resin suspension.

The solvent employed in the third composition may be any liquid which is inert to the system, is compatible with the other ingredients of the composition and which does not exert a solvating effect on the resinous particles. Suitable solvents are ketones such as acetone, methylethyl ketone, etc., aliphatic alcohols such as ethanol, propanol, isopropanol, diacetone alcohol, etc., halogenated hydrocarbons such as triand tetrachloroethylene, trichloroethane, etc.; hydrocarbons such as toluene, xylene, benzene, iso-octane, hexane, etc.; dimethyl formamide, dimethyl sulfoxide, -N-methyl pyrrolidone, dioxane, etc.

Isopropanol is the preferred solvent. Generally, the amount of solvent employed may vary from about 1 to about percent.

The aforedescribed coating compositions are separately applied to the electrical resistor successively in the stated order. The individual coated layers may be cured r or partially cured prior to the next subsequent coating operation. If desired, all three coating compositions may be individually deposited, and partially cured followed by an overall cure.

Any conventional coating method such as roller, dip, spray, etc., may be employed to coat the compositions of the invention.

A cure of the individual coatings is preferably effected through a cycle. In other words, the deposited coating is first heated for several minutes at about 80 C. to drive off the respective solvents. Thereafter, the temperature is gradually raised to about 250 C. and cooled to complete the cure.

A suflicient amount of the first or barrier composition is applied to the resistor to provide a final cured coating between V2 and 5 mils, preferably between 1 and 2 mils thick.

Suflicient amounts of the second or insulator coating composition are applied to yield a cured coating 5 to 10 mils, preferably 5 to 7 mils thick. It is preferred to apply two coats of the second composition, each having the above recited thicknesses.

Inasmuch as the third composition penetrates into the second composition, it does not contribute significantly to the thickness of the overall coating. Generally, suflicient amounts are added to increase the thickness up to about 2 mils, preferably 1 mil.

The invention will be illustrated by the following non-limiting examples. All percentages including those stated hereinabove and in the claims are by weight unless otherwise specified.

'EXAMPLE 1 3 coating compositions were prepared according to the following recipes:

(A) Barrier (first coating) Component: Parts by weight Polymethylphenyl siloxane 40 Isopropanol, anhydrous 50 Mica, white, waterground mesh) 7.5

(B) Insulator (second coating) (C) Impregnant (third coating) Component: Parts by weight Silbond H-4 (pre-hydrolyzed tetraethylorthosilicate) 8 Teflon suspension (MS-142C, Miller-Stephenson Co., 23% Teflon in toluene) Polymethylphenyl siloxane 5 Isopropanol, anhydrous 6 Glass-tin oxide 1K film resistors having a rated power of 1 watt were coated with the above compositions according to the following schedule:

Thickness, Minutes mils (1) Coated with A:

Solvent driven oil at 80-90 C Cured at 150 C Cured at 250 C Cooled to 100 C (2) Coated with B:

Solvent drivenoofi at 100 to 140 C Cooled to 100 C Coated with B again: Solvent driven off at 100-140 O Cured at 150 C Cooled to 100 C (4) Coated with C:

Solvent driven off at 90110 C Cured at 150 C Cured at 250 C Cooled to 100 C EXAMPLE 2 Example 1 was repeated except that Dow-Corning #991 Varnish (a 50% silicone resin in xylene dispersion wherein the silicone resin is one wherein the total silicon and oxygen content (as SiO is above 50% by weight of the resin) was substituted for the Glass Resin 100 in the barrier and impregnant compositions.

1K glass-tin oxide resistors having a rated power of 4 were coated as in Example 1 and similarly subjected to overloads of 100 times rated power and tested for moisture resistance.

No flaming or burning in the coating were observed upon overload and the resistors opened the circuit.

Moreover, the resistors were found to meet the requirements for maximum allowable AR.

EXAMPLE 3 Example 1 was repeated except that Dow-Corning R- 4471 (a silicone resin containing less than about 50% (as CiO Si and O) was substituted for Glass Resin 100.

SK glass-tin oxide film resistors having a rated power of 3 watts were coated as in Example 1 and subjected to an overload of 100 times rated power.

Actual flaming and burning of the coating resulted. Moreover, external arcing was observed and the resistor failed to open the circuit.

EXAMPLE 4 Example 1 Was repeated except that the following recipe was substituted for B, the insulator or second coating composition.

Component: Parts by weight Aluminum oxide 2160 Silicon dioxide (crystalline, 200 mesh) 540 Titanium dioxide (non-cowled, 300 mesh agglomerates) 453 Aerosil (amorphous silica) 10 Pigment (cobaltous aluminate-l-chromic oxide) 225 Pre-hydrolyzed tetrapropyl orthosilicate 915 Isopropanol 204 The resistors coated with the three compositions were subjected to overloads and moisture resistance tests as in Example 1 and found to be satisfactory.

EXAMPLE 5 Example 1 was repeated except that pre-hydrolyzed tetra-methyl orthosilicate was substituted for the tetraethyl ester.

The resistors coated with the three coating compositions were found to be satisfactorily flame-proof and moisture resistant.

EXAMPLE 6 Example 1 was repeated except that polyethylene was substituted for the polytetrafluoroethylene employed in the impregnant.

The resistors coated with the coating system were found to meet the Mil. Std. requirements for moisture resistance.

What is claimed is:

1. An article comprising a substrate coated with a. flame-proof, moisture resistant coating system comprising a plurality of cured coating compositions sequentially applied in the following order:

(1) a first coating composition consisting essentially of:

(a) from about 565% of a highly inorganic silicone resin containing at least about 50% by weight of Si and O as .SiO

(b) from about 110% inorganic filler, and

(c) from about 2070% of a compatible solvent;

(2) a second coating composition consisting essentially (a) from about 5 to 55% of an at least partially hydrolyzed tetraalkyl orthosilicate,

(b) from about 1 to 48% silicon dioxide,

(c) at least 4% aluminum oxide,

(d) from about 1 to 55% inorganic pigment,

(e) a suspension agent, and

(f) 020% of a compatible solvent; and

(3)fa third coating composition consisting essentially (a) from about 1 to 95% of an at least partially hydrolyzed tetraalkyl orthosilicate,

(b) from about 190% of a highly inorganic silicone resin containing at least 50% by weight of Si and O as SiO (c) from about 0.1 to 39% of resin particles having a low coeflicient of friction, and

(d) from about 1 to of a compatible solvent.

2. The article of claim 1 wherein said substrate is an electrical resistor.

3. The article of claim 1 wherein said substrate is an electrical film resistor.

4. The article of claim 1 wherein at least one of said silicone resins is derived from a member selected from the group consisting of difunctional and trifunctional silanes represented by the following respective structural formulas:

wherein R and R are selected from the group consisting of lower alkyl groups and a phenyl group and wherein R R and R are lower alkyl groups.

5. The article of claim 4 wherein said member is a difunctional silane, R is methyl and R is phenyl.

6. The article of claim 4 wherein said member is a trifunctional silane and said silicone resin is derived from two of said trifunctional silanes wherein R is methyl in one of said trifunctional silanes and phenyl in the other of said trifunctional silanes.

7. The article of claim 1 wherein at least one of said silicone resins is a polymethylphenyl siloxane.

8. The article of claim 1 wherein said filler is a silicate filler.

9. The article of claim 1 wherein said filler is a mica.

10. The article of claim 1 wherein the solvent of at least one of said coating compositions is isopropanol.

11. The article of claim 1 wherein the alkyl groups of at least one of said tetraalkyl orthosilicates are selected from the group consisting of a methyl, ethyl, propyl and butyl group.

12. The article of claim 11 wherein said group is an ethyl group.

13. The article of claim 1 wherein said tetraalkylsilicate exhibits a degree of hydrolysis of from -90%.

14. The article of claim 1 wherein said second coating composition includes titanium dioxide as an inorganic pigment.

15. The article of claim 1 wherein the silicon dioxide in said second coating composition has a size of from 100- 325 mesh.

16. The article of claim 1 wherein from 0.1 to about 2% of said suspension agent is present in the second coating composition.

17. The article of claim 16 wherein said suspension agent is an organic derivative of montmorillonite.

18. The article of claim 1 wherein said second coating composition includes up to 3% of a hardening agent.

19. The article of claim 1 wherein said coeflicient of friction is from 0.04-1.00 (static).

20. The article of claim 1 wherein said resin is a fluorocarbon resin.

21. The article of claim 1 wherein, after curing, said first coating composition has a total thickness of 0.5-5 mils, at least one layer of said second coating composition is present having a thickness of from 5-10 mils, and said third coating composition is up to about 2 mils thick.

22. The article of claim 1 wherein said resin particles in said third coating composition are present in the form of a suspension of resin particles comprising of 0.1 to 52% resin particles.

23. The article of claim 22 wherein from 1 to 75% of said susepnsion of resin particles is present in said third coating composition.

24. The process for coating a substrate comprising the steps of sequentially applying to said substrate the following coating compositions in the following order:

(1) a first coating composition consisting essentially of:

(a) from about 565% of a highly inorganic silicone resin containing at least from 50% by weight of Si and O as SiO (b) from about 1-10% inorganic filler, and

(c) from about -70% of a compatible solvent;

(2) a second coating composition consisting essentially (a) from about 5 to 55% of an at least partially hydrolyzed tetraalkyl orthosilicate,

(b) from about 1 to 48% silicon dioxide,

(c) at least 4% aluminum oxide,

(d) from about 1 to 55% inorganic pigment,

(e) a suspension agent, and

(f) 0-20% of a compatible solvent; and

(3) a third coating composition consisting essentially (a) from about 1 to 95% of an at least partially hydrolyzed tetraalkyl orthosilicate,

(b) from about 1-90% of a highly inorganic silicone resin containing at least 50% by weight of Si and O as SiO (c) from about 1 to 75 of a suspension of resin particles having a low coefficient of friction, and

(d) from about 1 to of a compatible solvent;

and

curing each of said coating compositions prior to the application of the next subsequent coating composition.

25. The process of claim 24 wherein said coating compositions are separately cured at a temperature of up to about 250 C.

26. An electrical resistor coated with a fiameproof, moisture resistant coating system comprising a plurality of coating compositions sequentially applied in the following order and cured:

(l) a first coating composition consisting essentially (a) from about 5-65% of a polymethyl phenyl siloxane containing at least about 50% by weight of Si and O as SiO (b) from about 1-10% of a mica, and

(c) from about 20-70% solvent;

(2) a second coating composition consisting essentially (a) from 5-55% of partially hydrolyzed tetraalkyl orthosilicate exhibiting a degree of hydrolysis of Iii-%,

(b) from 1-48% silicon dioxide,

(0) at least 4% aluminum oxide,

(d) from 1-55% of an inorganic pigment including from 1-45% titanium dioxide,

(e) from 0.1-2.0% of a suspension agent, and

(f) 0-20% solvent; and

(3) a third coating composition consisting essentially (a) from l-% of an at least partially hydrolyzed tetraalkyl orthosilicate exhibiting a degree of hydrolysis of 1090%,

(b) from 190% of a highly inorganic silicone resin containing at least 50% by weight of Si and O as SiOg,

(c) from 1-75% of a suspension containing from 0.1-52% resin particles having a coefiicient friction of 0.04-1.00% (static), and

(d) from 1-80% of a compatible solvent.

27. The article of claim 26 wherein, after curing, said first coating composition has a total thickness of 0.5-5 mils, at least one layer of said second coating composition is present having a thickness of from 5-10 mils, and said third coating composition is up to about 2 mils thick.

28. The process for coating a substrate comprising the steps of sequentially applying to said substrate the following coating compositions in the following order:

(1) a first coating composition consisting essentially (a) from about 5-65% of a highly inorganic silicone resin containing at least about 50% by weight of Si and O as SiO (b) from about 1-10% inorganic filler, and

(c) from about 20-70% of a compatible solvent;

1 1 (2) a second coating composition consisting essentially (a) from about 5 to 55% of an at least partially hydrolyzed tetraalkyl orthosilicate, (b) from about 1 to 48% silicon dioxide, (0) at least 4% aluminum oxide, (d) from about 1 to 55% inorganic pigment, (e) a suspension agent, and (f) 0-20% of a compatible solvent; and (3) a third coating composition consisting essentially of: i

(a) from about 1 to 95% of an at least partially hydrolyzed tetraalkyl orthosilicate,

(b) from about l-90% of a highly inorganic silicone resin containing at least 50% by weight of of Si and O as SiO;,

(0) from about 1 to 75% of a suspension of resin particles having a low coefficient of friction, and (d) from about 1 to 80% of a compatible solvent;

at least partially curing each of said first, second and third coating compositions prior to is at a temperature of up to about 250 C.

References Cited UNITED STATES PATENTS 5/1956 Radley 117-218 8/1962 Kohring 117-218 15 WILLIAM D. MARTIN, Primary Examiner R. HUSACK, Assistant Examiner US. Cl. X.R. 

